Four-cycle engines mounted in leisure vehicles such as motorcycles are required not only to exhibit high performance but also to have excellent external design appearance. In a water-cooled four-cycle engine, a water pump is disposed outside a crankcase to be apart from a cylinder block of the engine. An inlet port of the water pump is coupled to an outlet port of a radiator disposed behind a front wheel of the motorcycle through a rubber hose, and an outlet port of the water pump is coupled to an inlet port of a cooling water passage formed in the cylinder block of the engine through a rubber hose (see Japanese Laid-Open Patent Application Publication No. 2004-330990).
The rubber hose exposed outside may degrade external appearance of a motorcycle which is not equipped with a cowling, because the rubber is typically black and is noticeable. In addition, the rubber hose tends to wear out due to contact with other parts of the motorcycle during vibration of the engine.